The invention relates generally to the field of electro-optical devices and in particular to an optical sensor which provides an output signal as a function of a change in a sensed physical parameter such as pressure, temperature, absorption or motion.
Electro-optical devices have shown great promise in sensor applications. Passive optical sensors offer safe, accurate operation in hostile environments of heat and temperature, and are immune to electromagnetic interference. These advantages combine to make optically based sensors attractive for a number of applications.
A wide variety of optical sensing techniques have been suggested for measuring physical parameters. These devices however suffer from a lack of sensitivity, which is often required in modern sensors.
U.S. Pat. No. 4,475,812 issued to Buczek et al discloses an optical sensor with a gain medium situated at a first location, defining one end of a resonating optical cavity. An optical fiber couples electromagnetic radiation to a predetermined point at a second location, where it is directed toward a reflective surface, the reflective surface defining a second end of the resonating optical cavity. The physical condition to be sensed causes the reflective surface to move with respect to the end of the optical fiber. This movement essentially changes the length of the cavity in which the electromagnetic energy is resonating causing a corresponding change in the axial mode difference frequency within the cavity. Such a device requires a physical setup allowing for motion of the reflective surface with respect to the optical fiber, which may be difficult to accomplish. Furthermore, the sensitivity of such a sensor is limited by the ratio of the reflective surface travel to the total cavity length.
U.S. Pat. No. 4,775,214 issued to Johnson describes an optical sensor utilizing a single optical ring resonator having two independent resonant modes. The resonant frequencies of each of the resonant modes vary in different manners as the parameter to be measured changes. Such a device may be difficult to manufacture, in that it typically requires two independent resonant polarization modes whose resonant frequencies must vary in different manners in response to parameter changes. Furthermore, extraction of the parameter change requires complex characterization of the underlying sensor structure that is typically sensitive to process variations. The further requirement for a narrow bandwidth, variable frequency light beam source, adds additional cost and complexity, as such a source is sensitive to environmental changes and thus requires stabilization or control.
U.S. Pat. No. 6,278,811 issued to Hay et al. describes a fiber optic Bragg grating pressure sensor particularly suited for measuring ambient pressure of a fluid. Many sensor applications however are not centered on measuring the ambient pressure of a fluid, and a more flexible sensor is desirable.
U.S. Pat. No. 6,515,749 issued to Pipino discloses a chemical sensor, which includes an optical resonator including a nanostructured surface comprising a plurality of nanoparticles bound to one or more surfaces of the resonator. The nanoparticles provide optical absorption and the sensor further comprises a detector for detecting the optical absorption. The technique is somewhat limited to selective chemical interactions identifying the presence of target chemicals.
Thus there is a need for a method and apparatus combining high sensitivity with the ability to be adapted to a large variety of sensing applications.